The electronics industry uses particulate-based high-solids pastes in a variety of applications to ultimately provide conductive or dielectric capability in an electronic component. These pastes typically contain inorganic particles and an organic medium or vehicle. The inorganic particles provide the electrical functionality in the ultimate application, i.e., insulative or non-conducting inorganic particles form dielectric parts and conductive inorganic particles, primarily metals, form conductive parts. The organic medium facilitates application and control of the location of the inorganic particles.
After application of the paste, the organic medium is removed by heating and the inorganic particles are sintered or reflowed to form the electrically functional part. To apply the paste to the desired locations in the desired patterns, it is necessary that the paste flow well with the application method employed, such as stencil or screen printing, pressure dispensing etc.
A flow modifying agent or agents is typically included in the paste formulation for the paste to have the appropriate rheology or flow characteristics. Particularly desirable rheological characteristics of the paste include high viscosity, which precludes separation or settling of the inorganic particulate in the paste, dripping, stringiness, and slumping or sagging; plastic flow, which provides shear thinning with a well defined yield point for improved printing characteristics; sufficient tackiness to hold components; good transfer through the printing pattern; and clean release from the screen or stencil.
Effective amounts of flow modifiers, other organic ingredients, and acidic and/or basic activators, are typically so high that the organic ingredients leave a substantial residue on the solder after reflow. For example, rosin is typically present in a solder paste in the amount of by weight 25% or greater by weight in a solder paste vehicle. For thick film paste, ethyl cellulose, as a flow modifier, is typically present in the amount of 1-3% by weight of the paste. This amount of flow modifier leaves a substantial, readily visible and potentially solder-coating residue. It is necessary to wash off the residues since the residues contain various ionic contaminants and other chemical agents which can cause shorting and product failures over time.
Despite the undesirability of these residues, they have been of little concern until recently because chlorofluorocarbon solvents effectively and easily remove the residues. Today, however, considerable efforts are being made to replace such chlorofluorocarbon solvents due to their environmental impact upon the stratosphere. In addition, the failure of cleaning agents to completely remove residues from tight, hard-to-reach spots, and from under components in surface mount or fine pitch assembly operations, makes current methods and proposed alternative cleaners less effective.
Electronic pastes have been disclosed containing a polysaccharide, polysaccharide solvent and polyhydroxylated polysaccharide non-solvent, and optionally, an activator. However, these compositions have been known to be unsatisfactory due to screen plugging, slumping or poor print definition.
Therefore, it is an object of this invention to provide an electronic paste composition in which desirable rheological and flow properties of the paste are achieved with controllable intermolecular interactions. It is also an object of this invention to provide a vehicle for a solder paste which after the solder paste is reflowed leaves a residue that is negligible and innocuous so that no cleaning is necessary.